White light emitting LEDs (“white LEDs”) include one or more photoluminescence materials (typically inorganic phosphor materials), which absorb a portion of the blue light emitted by the LED and re-emit visible light of a different color (wavelength). The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor provides light which appears to the eye as being white in color. Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (100 lumens per watt and higher), white LEDs are rapidly replacing conventional fluorescent, compact fluorescent and incandescent lamps.
Various metrics exist for quantifying the characteristics and quality of light generated by white lighting sources. The two most commonly used metrics within the solid-state lighting industry are, Correlated Color Temperature (CCT) and International Commission on Illumination (CIE) General Color Rendering Index (CRI) Ra.
The CCT of a lighting source is measured in kelvin (K) and is the color temperature of a Planckian (black-body) radiator that radiates light of a color that corresponds to the color of the light generated by the lighting source.
The General CRI Ra characterizes how faithfully a lighting source renders the true colors of an object and is based on a measure of how well a lighting source's illumination of eight color test samples (R1 to R8) compares with the illumination provided by a reference source. In general, the higher the value indicates its closeness to a black radiator and natural sunlight. General CRI Ra can take negative values and has a maximum value of 100. Since the color samples R1 to R8 are all pastel colors (low saturation colors “Light Grayish Red” to “Reddish Purple”) the General CRI Ra gives a useful measure of subtle differences in light output of incandescent sources which generate a full spectrum that closely resembles sunlight. However, for white LEDs whose spectrum is composed of peaks, the General CRI Ra is proving to be inadequate as it is an average measure of color rendition over a limited range of colors and gives no information of the lighting source's performance for particular colors or highly saturated colors. Thus, when characterizing full spectrum solid-state white light emitting devices the CRI color samples R9 to R12 (saturated colors “Saturated Red”, “Saturated Yellow”, “Saturated Green”, “Saturated Blue”) and R13 to R15 (“Light Skin Tone”, “Leaf Green”, “Medium Skin Tone”) should be taken into account to give a meaningful characterization of full spectrum light.
To address the limitations of General CRI Ra, the Illuminating Engineering Society (IES) recently published the TM-30 standard for measuring and characterizing the color performance of lighting sources (Illuminating Engineering Society (2015) “Method for Evaluating Light Source Color Rendition” TM-30-15). Under the TM-30-15 standard, two metrics are used to characterize the color rendering characteristics of a lighting source, Fidelity Index (Rf) and Gamut Index (Rg). It is believed that IES TM-30-15 method has better correlation with people's color perception and, therefore, gives a more accurate characterization of a lighting source's light characteristics. The Fidelity Index Rf is similar to General CRI Ra and characterizes how faithfully a lighting source renders the true colors of an object and is based on a measure of how well a lighting source's illumination of 99 color samples compares with the illumination provided by a reference source. The Fidelity Index Rf value ranges from 0 to 100. The new color samples have been selected to be more representative of objects that are likely to be encountered in real-life applications and, as a result, the Fidelity Index Rf is believed to be a more accurate measure of color rendering than the General CRI Ra. Since Rf is measured over a greater number of color samples, it will be more difficult to achieve high scores compared with the General CRI Ra. Moreover, due to the different testing procedures, General CRI Ra and Fidelity Index Rf values are not comparable against each other.
The Gamut Index Rg focuses on color saturation and is an average level of saturation compared with a reference source. The Gamut Index correlates to the vividness of the appearance of colored objects. The Gamut Index Rg value ranges from 60 to 140 where values below 100 indicate decreased saturation and values above 100 indicate increased saturation compared with the reference source.
A further problem with known white LEDs, that are commonly used in display backlights of cell phones, is the damage they potentially cause to the human eye and particularly in view of ever-increasing screen-time usage. The American Macular Degeneration Foundation (AMDF) have reported that the blue rays of the spectrum appear to accelerate age-related macular degeneration (AMD) more than any other rays in the spectrum. High Energy Visible (HEV) blue light in the region of 400 nm-500 nm has been identified for years as the most dangerous light for the retina. Almost all visible blue light passes through the cornea and lens and reaches the retina. This light can affect vision and can prematurely age the eyes. Early research shows that too much exposure to blue light (HEV) can lead to digital eyestrain and retinal damage. This can cause vision problems such as age-related macular degeneration. This damage occurs when blue light (HEV) penetrates the macular pigment of the eye and causes a breakdown of the retina, leaving the eye more vulnerable to blue light exposure and cell degeneration.
The present invention arose in an endeavor to improve the color rendition of full spectrum light emitting devices comprising photoluminescence conversion materials that generate white light having high color rendering characteristics such as a high CRI Ra and/or high Fidelity index Rf. The present invention also ameliorates problems associated with White LEDs causing damage to the human eye by exposure to HEV as discussed above.